The present invention is directed to a pressure sensitive adhesive composition comprised of a polymerized acrylic or methacrylic acid ester backbone having grafted thereto pendant polymeric moieties.
Polymeric compositions are known which are comprised of backbone polymers having grafted thereto pendant polymeric moieties. The type of backbone polymer and graft polymeric moiety employed varies depending upon the desired characteristics of the end product. See, for example, U.S. Pat. Nos. 3,786,116; 3,832,423; 3,842,146; 3,862,077; 3,879,494; 3,928,255; 3,989,768; 4,085,168; 4,551,388; 4,554,324; 4,656,213; 4,693,776; 4,732,808; and 4,871,812 which disclose various types of such polymers which may or may not exhibit pressure sensitive adhesive properties.
Typical of the type of polymeric compositions disclosed in the above patents are compositions comprised of a backbone polymer such as an acrylic or methacrylic backbone polymer having attached thereto a graft polymer comprised of a polymerizable macromolecular monomer such as styrene or alpha-methylstyrene. See, for example, U.S. Pat. No. 4,554,324, among others, in this regard.
The acrylic pressure sensitive adhesives such as described in U.S. Pat. No. 4,554,324 may be made from an acrylic ester and a polar acrylic monomer. The polar acrylic monomer can be one or a mixture of acrylic acid, acrylamide, acrylonitrile, itaconic acid, etc. The acrylic ester can be any aliphatic ester of acrylic acid. Such monomers are typically polymerized free radically by solution, suspension or emulsion polymerization. The acrylate portion of the copolymer is generally present in a generally high concentration and renders the polymer tacky. The polar monomer increases the ability of the adhesive to bond to a surface.
While such polymeric compositions have been found to have utility as pressure sensitive adhesives, one area that has not heretofore been addressed with much success is providing high performance pressure sensitive adhesives which may be suitable for use in high temperature environments (i.e., environments where the temperature exceeds about 250.degree. F.).
By way of example, the aerospace and automotive industries are experiencing an increased demand for adhesives with the capability to withstand temperature extremes of -200.degree. F. to as high as 400.degree. F. It has been found that typical acrylic pressure sensitive adhesives, while performing satisfactorily at temperatures as low as -45.degree. F., fail at temperatures of about 150-200.degree. F. (and sometimes at lower temperatures) as the adhesive becomes too soft and is unable to retain acceptable adhesive properties. Attempts have been made to enhance the performance of such adhesives by cross-linking procedures.
Presently, silicone pressure sensitive adhesives are known for their high performance characteristics including their ability to perform at temperatures as high as 500.degree. F. However, the range of end uses for such adhesives is much more limited than acrylic pressure sensitive adhesives. Unfortunately, acrylic pressure sensitive adhesives are not able to satisfactorily perform at the same high temperatures as silicone adhesives.
The problems which exist in prior art acrylic adhesive compositions in connection with high temperature stability results from the fact that as the temperature of use exceeds the glass transition or melt temperature of the reinforcing graft or "hard" segment of the polymer composition, both the "hard" segment and the "soft" acrylic polymer backbone tend to flow. Such flow tendency adversely effects the ability of the composition to serve as an adhesive at high temperatures.